1. Field of Invention
The present invention relates to the measuring method and circuit for electrical characteristics, and in particular to the measuring method and circuit for, current-voltage characteristics.
2. Description of Related Art
Conventionally, the measurement of the device parameters under the static bias is affected by the factors of self-heating effect, causing a difference between the measured result and the ideal value. The self-heating effect is a phenomenon that the device current decreases due to the decrease of mobility of electrons in the device when the device temperature rises. Particularly, the self-heating effect is more obvious for the high bias or high power devices such as Laterally Diffused Metal Oxide Semiconductor (LDMOS) devices and Silicon On Insulator (SOI) devices.
As an example to illustrate the self-heating effect conveniently, all the to-be-measured devices in the following drawings are N-channel Metal Oxide Semiconductor (NMOS) transistor. FIG. 1 is a characteristic curve for drain current versus drain-source voltage of the transistor with/without self-heating effect. It is evident that the self-heating effect causes the decrease of drain current.
FIG. 2A is a circuit drawing, schematically illustrating a conventional measuring circuit for the current-voltage characteristics of transistor. The measuring process is first, applying a constant voltage VIN to the gate of the transistor 201. Next, by changing the voltage of power source VDD and measuring the voltage VOUT between the drain and the source, the drain current Id=(VDD−Vout)/R can be calculated out. Thus, the characteristic curve for the drain current versus the drain-source voltage Vout under the applying the gate voltage can be obtained. The shortcoming of this method is that the self-heating effect causes the drain current to decrease for the high power circuit.
In order to reduce the self-heating effect, the disclosure in EEE Electron Device Letters Vol. 16 No. 4 pp. 145-147 has provided an equivalent circuit to the circuit of FIG. 2A of the measuring circuit for measuring the current-voltage characteristic curve of a transistor except that the clock signal as shown in FIG. 2B is instead of a static voltage applied to the gate. The measuring process is first applying a clock signal Vin to the gate of transistor 201. Next, by changing the voltage of the power source VDD and measuring drain-source voltage Vout, then the drain current Id=(VDD−Vout)/R can be calculated to obtain the characteristics curve of drain current versus drain-source voltage under the gate voltage Vg. In order to obtain the intended precision for the current, the resistance R should be large in selection. In above measuring process, even though the temperature gradually rises due to the transistor being at high bias when the clock signal is at the high voltage level (HI), the temperature gradually decreases due to the transistor being at the off state by the clock signal at the low voltage level. Therefore, it is possible to reduce the self-heating effect by a proper duty cycle, which is the ratio of the quantity of time of the clock signal HI to the quantity of time of the clock period T, since the heat accumulation on the transistor is not as fast as the prompt change of clock signal. A shortcoming of above measuring method is that a breakdown may occur on the transistor. For example, in order to let the Vout be the maximum operation voltage od the transistor, VDD=Vout+Id*R. When the clock signal is al low voltage level (LO), the drain electrode has the VDD=Vout+Id*R, then it cause the breakdown on the junction between the drain electrode and the substrate.